While Glass's voice is never heard on the calls, the calls do pick up a voice, believed to be that of his murderer.

"Whether or not you're a dead man. Just hang up," the voice says.

Glass was serving multiple six-year sentences for possession of a firearm after a felony conviction and possession of drugs in jail.

When prison guards found him dead his body has visible injuries. Police are awaiting toxicology reports to determine an exact time of death.

Police Captain Craig Akard said assaults can happen frequently at prisons but that it is unusual for them to result in death.

"You may have a group of guys involved when a skirmish starts," he said. "Maybe the same guy gets into a fight with another the next day, but both parties walk away ... You can't really say 'that's where the (fatal) injury occurred.'"